Ab Initio Investigation of Li and Na Migration in Guest-Free, Type I Clathrates

Abstract

Guest-free, type I clathrates with formula Tt46 (Tt = Si, Ge, Sn) are comprised of open, cage-like frameworks with the potential for facile Li or Na conduction. Herein, ab initio density functional theory (DFT) is used to evaluate the ionic mobility of Li and Na through the clathrate crystal structures. The favorable Li and Na positions inside the clathrate structures are determined, and the migration pathways and barriers are evaluated using the nudged elastic band (NEB) method. The results show that it is energetically favorable for a Li atom to occupy the center position inside the small Tt20 cages while preferring the off-center positions in the larger Tt24 cages. The lowest Li migration barriers are found to be 0.35, 0.13 and 0.37 eV for Si46, Ge46, and Sn46, respectively, with the dominant diffusion pathway along channels of Tt24 cages connected by hexagonal faces. Li accessibility to the Si20 cage in Si46 appears to be restricted in the dilute regime due to a high energy barrier (2.0 eV) except for...